I know you said no Kindle, but the Kindle DX is really the best for textbooks due to the amount information that can be displayed. You can easily use a free program like Calibre to convert pdf.

Have you considered a tablet? Battery life may not be great but it is easy to make notes and will read PDFs well.

I personally use my Kindle 3 for PDF textbooks and have never had an issue but I don't make that many annotations, which could be an issue with the 5-way direction buttons. If I were getting another e-reader for studying I would consider a colour one due to the large amount of diagrams my subject required but I guess linguistics is not diagram heavy.

Yes, this is my point. to make many annotations with the five way direction button is a real pain and thats why I want to get another device.